Mechanisms of active folding of the landscape (southern Tian Shan, China)

We explore the kinematic mechanisms of active large‐scale folding, based on analysis of two adjacent major anticlines in Tian Shan (central Asia) that share an acceleration of shortening rate leading to topographic emergence and folded geomorphic surfaces. Their folding mechanisms are fundamentally different. Yakeng anticline is a gentle pure shear detachment fold with 1200 m of shortening and a well‐constrained history of growth beginning at 5.5 Ma with an order‐of‐magnitude increase in shortening rate from 0.16 to ∼1.2–1.6 mm/yr at ∼0.16–0.21 Ma. The shape of the deformed topographic surface and of subsurface horizons deposited during deformation is a linearly proportional image at reduced amplitude of the deeper structure, which shows that instantaneous uplift rates have been pointwise linearly proportional to the current finite fold amplitude. In contrast, Quilitak anticline is a complex fault bend fold with uplift rates proportional to the sine of the fault dip, showing discontinuities in uplift rate across active axial surfaces. The 10‐ to 20‐km‐wide anticline is topographically emergent only in a central 5‐ to 7‐km‐wide mountainous uplift, the abrupt southern edge of which is marked by ∼600‐ to 700‐m‐high triangular facets that result from active folding of a pediment across an active axial surface. The giant facets are shown to form by kink band migration and record postemergence deformation since an order‐of‐magnitude acceleration in shortening rate from ∼0.6 to ∼4–5 mm/yr, apparently contemporaneous with Yakeng. Sections logged across the active ∼115‐m‐wide hinge zone show that recent strata provide a bed‐by‐bed record of fold scarp growth, which is quantitatively deciphered by fitting bed shapes to a finite width kink band migration model.